Health monitoring devices, such as fitness and wellness devices, may be capable of non-invasively measuring a variety of physiological characteristics of a subject via optical sensing. Such health monitoring devices can include a light source and an optical sensor.
The light source can illuminate a portion of a measurement site or even emit light that penetrates beneath the measurement site, such as into the stratum corneum of the skin or into blood vessels beneath the skin. Light from the light source may be scattered, absorbed, and/or reflected throughout the measurement site or the material forming the measurement site, such as the skin. The amount of scatter, absorption, or reflection can depend directly or indirectly on one or more physiological characteristics of the measurement site.
The optical sensor can collect light exiting the measurement site and generate electrical signals corresponding to the collected light, which may be conveyed (in the form of electrical signals) as information or data to the health monitoring device. The health monitoring device can use the optical sensor data to extrapolate, determine, derive, estimate, or measure physiological parameters of the measurement site.
In many cases, the optical sensor data may include noise associated with ambient light, surface conditions of the measurement site (e.g., cleanliness, hair, perspiration, etc.), proximity of the optical sensor and/or light source to the measurement site, and motion artifacts caused by the relative motion between the health monitoring device and the measurement site.
Furthermore, health monitoring devices often have a small form factor and are wearable by a subject for extended periods of time. The constrained proportions of such devices can limit the maximum physical size of the optical sensor and/or light source, effectively restricting the performance of both. For example, smaller light sources may emit less light and smaller optical sensors may detect less light. In addition, as the size of the optical sensor and/or light source decrease, the effects of noise increase. As a result, the accuracy, precision, and/or reliability of the physiological parameters derived from the optical sensor data can decrease with the size of many current health monitoring devices.
Accordingly, there may be a present need for an improved optical sensing system configured for use with a small form factor health monitoring device.